1. Field of the Invention
This invention relates to a pixel signal correcting device which helps to eliminate non-uniformity in pixel signals emitted from photoelectric transfer element arrays and used for calculating defocus amount, etc.
2. Related Background Art
In the focus detecting device of a camera of the type which obtains defocus amount to effect automatic focusing, defocus amount calculation has conventionally been carried out after A/D-converting pixel signals emitted from a plurality of photoelectric transfer element arrays.
Theoretically, the output levels of pixel signals emitted from photoelectric transfer element arrays for focus detection should be uniform in the case of an object of a uniform luminance. In practice, however, it is common for the output levels not to be uniform as shown for example in FIG. 5, due to the unevenness in sensitivity of the photoelectric transfer element arrays, the influence of the vignetting factor of the focus-detection optical system for leading light to the photoelectric transfer element arrays, etc. To eliminate this non-uniformity in pixel signals, it has conventionally been the practice to previously store, in a semiconductor memory, correction values respectively corresponding to the pixels, and to perform pixel signal correction in accordance with the correction values thus stored, thereby obtaining uniform pixel signals as shown in FIG. 6.
This correcting calculation is carried out by using, for example, the following formula: EQU AD(n).times.(1+Q(n)/256) (1)
where AD(n) is the A/D-converted value of a pixel output, and Q(n) is a corresponding correction value stored in a semiconductor memory.
The correction value Q(n) consists of signed 8-bit data, which allows a maximum correction of .+-.50%. The correction value Q(n) is determined as follows: First, a surface of a uniform luminance is presented to the photoelectric transfer element arrays to obtain pixel signals therefrom. Then, a pixel is chosen as a reference value, and the differences between this reference value and the pixel signals are obtained as standardized values.
Thus, with such a conventional pixel signal correcting device, each correction value is obtained from variation with respect to a reference value of pixel signals obtained by presenting a uniform-luminance surface to photoelectric transfer element arrays. Accordingly, the variation can generally range from a negative to a positive value, and, consequently, the corresponding correction value which is to be stored in the semiconductor memory also ranges from negative to positive. As a result, the correcting calculation represented by the above formula (1), which is performed by using a microprocessor, necessitates a routine for sign judgment. Since each correction value exhibits a positive or a negative sign and sign judgment has to be made for each pixel, the requisite time for image processing is inevitably rather long.